The first step in viral infection. Attachment of a virus to its host is very specific and is also known as adsorption.

Autotroph

An organism that can makes its own food, typically using C02 as a carbon source.

Auxotroph

A bacterium that cannot) survive on minimal medium (glucose alone) because it lacks the ability to synthesize a molecule it needs to live (typically an amino acid). Auxotrophs must have the needed substance (the auxiliary trophic substance) added to their medium in order to survive. They are typically denoted by the substance they require followed by a "-" sign in superscript. For example, a bacterium that cannot synthesize leucine would be a leucine auxotroph, and would be indicated as Leu-

Bacillus

A bacterium having a rod-like shape (plural = bacilli)

Bacteriophage

A virus that infects a bacterium.

Binary fission

An asexual method of bacterial reproduction that serves only to increase the size of the population; there is no introduction of genetic diversity. The bacterium simply grows in size until it has doubled its cellular components, then it replicated its genome and splits into two.

Capsid

The outer protein coat of a virus.

Chemotaxis

Movement that is directed by chemical gradients, such as nutrients or toxins.

Chemotroph

An organism that relies on a chemical source of energy (such as ATP) instead of using light to make ATP (like phototrophs do).

Chitin

A polysaccharide found in the cell walls of fungi and in the exoskeletons of insects.

Coccus

A bacterium having a round shape (plural = cocci)

Conjugation

A form of genetic recombination in bacteria in which plasmid and.or genomic DNA is transferred from one bacterium to the other through a conjugation bridge.

Endotoxin

A normal component of the outer membrane of Gram-negative bacteria. Endotoxins produce extreme immune reactions (septic shock), particularly when many of them enter the circulation at once.

Envelope

A lipid bilayer that surrounds the capsid of an animal virus. The envelope is acquired as the virus buds out through the plasma membrane of its host cell. Not all animal viruses possess an envelope.

Excision

The removal (and usually the activation) of a viral genome from its host's genome.

Exotoxin

A toxin secreted by a bacterium into its surrounding medium that help the bacterium compete with other species. Some exotoxins cause serious diseases in humans (botulism, tetanus, diphtheria, toxic shock syndrome).

Facultative Anaerobe

An organism that will use oxygen to produce energy (aerobic metabolism) if it is available, and that can ferment (anaerobic metabolism) if it is not.

F (fertility) factor

A bacterial plasmid that allows the bacterium to initiate conjugation. Bacteria that posses the F factor are known as F+ "males."

Flagella

A long, whip-like filament that helps in cell motility. Many bacteria are flagellated, and sperm are flagellated.

Gram-negative bacteria

Bacteria that have a thin peptidoglycan cell wall covered by an outer plasma membrane. They stain very lightly (pink) in Gram stain. Gram-negative bacteria are typically more resistant to antibiotics than Gram-positive bacteria.

Gram-positive bacteria

Bacteria that have a thick peptidoglycan cell wall, and no outer membrane. They stain very darkly (purple) in Gram stain.

Heterotoph

An organism that cannot make its own food, and thus must ingest other organism.

Hfr Bacterium

High frequency of recombination bacterium. An F+ bacterium that has the fertility factor integrated into its chromosome. When conjugation takes place, it is able to transfer not only the F factor, but also its genomic DNA.

Lag phase

A short period of time prior to exponential growth of a bacterial population during which no, or very limited, cell division occurs.

Lawn

A dense growth of bacteria that covers the surface of a Petri dish.

Lipid

A hydrophobic molecule, usually formed from long hydrocarbon chains. The most common forms in which lipids are found in the body are as triglycerides (energy storage), phospholipids (cell membranes), and cholesterol (cell membranes and steroid synthesis).

Log phase

The period of exponential growth of bacterial population.

Lysogenic cycle

A viral life cycle in which the viral genome is incorporated into the host genome where it can remain dormant for an unspecified period of time. Upon activation, the viral genome is excised from the host genome and typically enters the lytic cycle.

Lysozyme

An enzyme the lyses bacteria by creating holes in their cell walls. Lysozyme is produced in the end stages of the lytic cycle so that new viral particles can escape their host; it is also found in human tears and human saliva.

Lytic cycle

A viral life cycle in which the host is turned into a "virus factory" and ultimately lysed to release the new viral particles.

Medium

The environment in which or upon which bacteria grow. It typically contains a sugar source and any other nutrients that bacteria may require. "Minimal medium" contains nothing but glucose.

Mutualism

A form of symbiosis in which both organisms involved benefit from the association.

Obligate aerobe

An organism that requires oxygen to survive (aerobic metabolism only).

Obligate anaerobe

An organism that can only survive in the absence of oxygen (anaerobic metabolism); oxygen is toxic to obligate anaerobes.

Parasite

An organism that requires the aid of a host organism to survive, and that harms the host in the process.

Penetration

The second step in viral infection, the injection of the viral genome into the host cell.

Peptidoglycan

A complex polymer of sugars and amino acids; the substance from which bacterial cell walls are made.

Periplasmic space

The space between the inner and outer cell membranes in Gram-negative bacteria. The peptidoglycan cell wall is found in the periplasmic space, and this space sometimes contains enzymes to degrade antibiotics.

Phototroph

An organism that utilizes light as its primary energy source.

Pilus

A long projection on a bacterial surface involved in attachment, e.g. the sex pilus attaches F+ and F- bacteria during conjugation.

Plaque

A clear area in a lawn of bacteria. Plaques represent an area where bacteria are lysing (dying) and a usually caused by lytic viruses.

Plasmid

A small, extrachromosomal (outside the genome), circular DNA molecule found in prokaryotes.

Productive cycle

A life cycle of animal viruses in which the mature viral particles bud from the host cell, acquiring an envelope (a coating of lipid bilayer) in the process.

Prokaryote

An organism that lacks a nucleus or any other membrane-bound organelles. All prokaryotes belong to either Domain Bacteria or Domain Archea (formerly Kingdom Monera)

Retrovirus

A virus with an RNA genome (e.g. HIV) that undergoes a lysogenic life cycle in a host with a double-stranded DNA genome. In order to integrate its genome with the host cell genome, the virus must first reverse-transcribe its RNA genome to DNA.

Reverse transcriptase

An enzyme that polymerizes a strand of DNA by reading an RNA template (an RNA dependent DNA polymerase); used by retroviruses in order to integrate their genome with the hose cell genome.

RNA dependent RNA polymerase

A viral enzyme that makes a strand of RNA by reading a strand of RNA. All prokaryotic and eukaryotic RNA polymerases are DNA dependent; they make a strand of RNA by reading a strand of DNA.

Saprophyte

An organism (such as fungus) that feeds off dead plants and animals.

Spirochete

A bacterium having a spiral shape (plural = spirochetes).

Tolerant anaerobe

An organism that can survive in the presence of oxygen (oxygen is not toxic), but does not use oxygen during metabolism (anaerobic metabolism only).

Transduction

The transfer by a lysogenic virus of a portion of a host cell genome to a new host.

Virus

A nonliving, intracellular parasite. Viruses are typically just pieces of nuclei acid surrounded by a protein coat.